A center pivot irrigation system employing an end gun sprinkler must typically deliver oil to drive hydrostatic motors under pressures in the 1,500 P.S.I. to 2,000 P.S.I. range. In addition to the high pressure hose a return hose must carry oil back to a reservoir, usually located at the pump. Before the oil enters the reservoir, it passes through a restrictor valve which maintains 65 P.S.I. in the return line. This relatively low pressure is used to pressurize an end gun control circuit.
A typical end gun control circuit consists of a small low pressure line running from the reservoir the length of the system to a diaphragm valve which is utilized to turn the end gun sprinkler on and off. This diaphragm valve is controlled in a programmed manner by a conventional three-way control valve which is actuated by cams and located at the pivot point. When the control valve is open the circuit is open directly back to the reservoir. When the valve is actuated, the low pressure is applied to the diaphragm valve. Accordingly, the end gun sprinkler may be shut off as the system passes by roads or other areas where water is not desired. Generally, the low pressure is applied to the end gun sprinkler only about 20% of the time.
Because the pump, reservoir, restricter valve and end gun control valve are stationary and located at the pivot point, three hoses, a very high pressure line, a return line at low pressure and a low pressure line follow the sprinklers as they rotate about the pivot point. These lines will continually be wrapped around the pivot point as the system rotates unless other provisions are made.
A common solution is to provide flexible hoses of sufficient length to permit several revolutions before the system must be stopped and the hoses disconnected. Then the hoses are typically rewrapped around the pivot point in a direction opposite to that which the system will travel and then reconnected. If the operator forgets to rewrap the hoses or the hoses catch on an obstruction during operation, they may be pulled apart and, as a consequence, the hydraulic oil is lost and the hydraulic pump may suffer severe damage.
Another solution which has been attempted is to use hydraulic swivel joints. Hydraulic swivel joints, which form circular flow channels between a stationary element and a rotating element, are well known. Originally such joints were designed to define a high pressure constant flow port and a low pressure port; however, additional ports have been subsequently added. Such multi-port swivel joints were originally designed for use in high speed applications and where frequent observation and maintenance is practical.
Such swivel joints generally employ neoprene O-rings, or the like, which function satisfactorily in applications where moderate to high speeds are encountered, and the speed of the rotating member helps maintain seal contact. However, center pivot irrigation systems may revolve no more than one time in each 20 to 120 hours. Under these conditions, neoprene O-rings and their derivatives have a high tendency to stick to one or both members of the swivel. When this sticking occurs, the O-rings tear, bunch or wad. Also, if such O-ring seals are initially defective, or are damaged even slightly during installation or assembly, a leak is guaranteed. Any flaw, regardless of how derived, will almost instantly become an unmanageable leak under pressures of the magnitude encountered in a center pivot irrigation system. This problem is magnified by the pulsations from variable displacement pumps which are widely used in center pivot irrigation systems.
Heretofore, in a typical three port hydraulic swivel joint, the highest pressure is distributed from the center port in order that those ports which are under less pressure will be adjacent to the exterior. By use of this configuration, the seals which separate the interior of the swivel joint from the environment would ordinarily have a better chance of keeping the oil within the joint and contamination outside of it. However, there is a major disadvantage of this configuration when used in a center pivot irrigation system. If the seal which separates the center high pressure port and the end gun sprinkler port should leak, high pressure oil will enter the end gun port. If the end gun circuit is open there is no problem since this oil will return to reservoir. However, if the end gun circuit is closed, the high pressure oil will enter the 65 P.S.I. end gun circuit and either rupture the small end gun line or possibly ruin the diaphragm valve.